In order to solve the problem that special users’ energy saving potential is unable to diagnose automotive online, a new automatic diagnosis of energy saving potential method based on online DSP was proposed. This proposed method, together with a variety of techniques like modern power electronics, digital signal processing, high precision and fast sampling, high capacity storage, human-machine interaction technologies and so on, was applied to develop a corresponding special users energy saving potential diagnosis detector. The hardware design and DSP energy saving analysis software design of the detector were described firstly, and the results of field test were presented to demonstrate its feasibility. It is shown that, its advantages include easy-to-use, low cost, highly reliable, strong intelligence and easy to promote, thus can effectively improve the efficiency of electrical energy audit and the degree of information and automation.